The present invention generally relates to telecommunication systems, and more particularly to a telecommunication network that provides distributed control for restoring a communication failure in the network automatically.
In a telecommunication network that includes a plurality of nodes connected with each other by links, a communication path is established through a number of nodes, and information is passed between the nodes as well as through links that connect the nodes with each other. When a failure occurs in one or more of the nodes or links that form a communication path, an alternative communication path has to be searched for in order to maintain the connection. It should be noted that such en alternative path has to hive a sufficient transmission capacity for maintaining the information traffic.
Conventionally, two different control processes are known for performing the alternative path search, one being known as a centralized process and the other being known as a distributed process. The centralized process uses a central station provided commonly to the nodes that are included in the network. In the centralized process, the central station conducts a search for an alternative path that avoids a defective node or link upon detection of a failure in one of the nodes or links forming a communication path. In such a centralized process, the central station is required to conduct an extensive and complex searching operation of the alternative path for numerous combinations of the nodes, and the time necessary for restoring communication inevitably becomes long. Needless to say, one has to provide an extensive processing facility at the central station.
The distributed process, on the other hand, activates a so-called flooding process at first at the node that is included in the existing communication path and located adjacent to the defective node or link. There, the node activates a broadcast with a message indicative of the search for an alternative path originating from a first end node and extending to a second end node, together with information that designates the node that has issued the message as being the second end node of the alternative path. There, the message is transmitted to all the nodes that are linked to the second end node, and each node that has received the broadcasted message in turn activates a re-broadcasting with a message with the information for designating the node that has caused the rebroadcast stamped on the message. When the first end node receives the broadcast after one or more re-broadcastings, the first end node can identify the path that the message has been transmitted on by examining the information about the nodes that are included in the message. Thus, by sending the information along the path thus identified in the direction opposite to the direction of the broadcast, one can maintain the connection while avoiding the defective node.
FIG. 1 shows the foregoing distributed control process conducted in a network to search for the alternative path.
Referring to FIG. 1 showing a part of the network, the network includes a number of nodes N1-N9 wherein a communication path passing through the nodes N2, N5 and N8 is first established for carrying information traffic that flows from the node N2 to the node N8 via the node N5 or vice versa. When the node N5 becomes defective, for example, this fact is detected immediately at the node N2 or N8 that is located adjacent to the defective node N5, and one of the nodes, such as the node N8, activates a broadcast to all the nodes that are linked thereto. There, the node N8 transmits a path-search message in search of the alternative path, wherein the node N8 is identified in the message as an end node of the alternative path and the node N2 is identified as another end node of the alternative path. In response to the broadcast from the node N8, each of the nodes adjacent to the node N8, such as the nodes N4, N6, N7, N9, . . . , re-broadcasts the path-search message with the information identifying the node, such as the node N4, that causes the re-broadcasting being stamped on the path-search message. By repeating the foregoing broadcasting process, the path-search message ultimately reaches the end node N2 with the information of the nodes, such as the node N4, through which the path-search message has been relayed. Based upon the path-search message thus received, the node N2 transmits a message for setting up an alternative path that avoids the defective node N5 to the relaying node N4 for affecting a switching of communication path therein, and the node N4 in turn transmits a message for effecting a switching of the communication path to the node N8. Thereby, an alternative communication path is setup between the node N2 and the node N8.
When restoring communication according to the alternative path thus searched, it is necessary to rewrite the interconnection switching table, called VPI (virtual path identifier) table, that is provided in each node for specifying the cross-connection between incoming transmission paths and outgoing transmission paths so that the information transmitted from an end node of the alternative path reaches the other end node. According to the CCITT protocol, there are in all 4096 virtual path identifiers in each link, and the VPI table therefore contains a matrix for cross-connecting 4096 virtual path identifiers as represented in TABLE I, wherein TABLE I shows the VPI table for a node, such as the node N8, that switches the incoming virtual paths 0001, 0002, 0003, 0004 . . . to outgoing virtual paths 0001, 0002, 0003, 0004, . . . respectively, where the numerals such as 0001, 0002, 0003, 0004 . . . represent the virtual path identifier.
TABLE I ______________________________________ IN OUT ______________________________________ 0001 0001 0002 0002 0003 0003 0004 0004 . . . . . . 4906 4906 ______________________________________
It should be noted that each node has its own VPI table that differs from other nodes for establishing a predetermined communication path. Thus, it will be understood that the VPI table has to be modified when restoring communication and that the information for modifying the VPI table has to be transmitted to each of the nodes that are included the alternative path. As the VPI table includes 4096 items, the transmission of the whole VPI table to of the nodes included in the alternative path requires transmission of a very large amount of information. The transmission of such a large amount of information through low-speed control channel of the network requires a considerable amount of time, and the time needed for establishing the alternate path becomes inevitably long.